5 ms·
How? Unless you have an atomic clock nearby, they will very quickly drift apart by many nanoseconds again. It's also impossible to synchronize to that level of
by Asraelite 2y ago
How? Unless you have an atomic clock nearby, they will very quickly drift apart by many nanoseconds again. It's also impossible to synchronize to that level of precision across a network.
- AlotOfReading 2y agoIt's not only possible, you can demonstrate it on your phone. Check the GPS error on your device in a clear area. 1 ft of spatial error is roughly 1ns timing error on the signal (assuming other error sources are zero). Alternatively, you can just look at the published clock errors: http://navigationservices.agi.com/GNSSWeb/PAFPSFViewer.aspx http://navigationservices.agi.com/GNSSWeb/PAFPSFViewer.aspx All the satellites in all of the GNSS constellations are synchronized to each other and every device tracking them to within a few tens of nanoseconds. Yes, atomic clocks are involved, but none of them are corrected locally and they're running at a significantly different rate than "true" time here on earth.
- Asraelite 2y agoThat's true, but it's not really the situation I'm thinking of. Your phone is comparing the differences between the timestamps of multiple incoming GNSS signals at a given instant, not using them to set its local clock for future reference. A better analogy to practical networked computing scenarios would be this: receive a timestamp from a GNSS signal, set your local clock to that, wait a few minutes, then receive a GNSS timestamp again and compare it to your local clock. Use the difference to measure how far you've travelled in those few minutes. If you did that without a local atomic clock then I don't think it would be very accurate.
- AlotOfReading 2y agoThat's a common way of doing high precision time sync, yes. It's slightly out of phone budget/form factor, but that's what a GPSDO does. The receiver in your phone also needs pretty good short term stability to track the signal for all of the higher processing. It'd be absolutely fine to depend on PPS output with seconds or minutes between measurements.
- wat10000 2y agoBasic hardware gets you a precise GNSS time once per second. Your local clock won’t drift that much in that time, and you can track and compensate for the drift. If you’re in a position to get the signal and have the hardware, then you can have very accurate clocks in your system.
- tucnak 2y agoUntil somebody start spoofing GPS like they do in Ukraine, and you look embarrassing.
- AlotOfReading 2y agoSo use Galileo's OSNMA instead. That'll work until they spend $100 on a jammer.
- tucnak 2y agoI hate to break it to you, but all modern electronic warfare equipment has been targeting all GNSS for many years now. There's a reason why "GPS-denied", which is really referring to any form of satellite navigation, is a multi-billion dollar industry.
- AlotOfReading 2y agoGNSS does in fact work fairly well even in conflict zones, but I'm not sure why you're trying to make this point. Everything is vulnerable to some level of warfare and civilian infrastructure is almost never designed with the expectation that it'll operate in a conflict zone. Are you suggesting we individually airgap all the networking equipment with local atomic clocks behind 10m of specially reinforced concrete instead? Even the most secure civilian facilities (data centers) are fine sticking 2-3 receivers on the roof and calling it good.
- mgaunard 2y agoWhiteRabbit achieves sub-nanosecond time synchronization over a network.
- prerok 2y agoThe Precision Time Protocol is intended to solve this problem: https://en.m.wikipedia.org/wiki/Precision_Time_Protocol https://en.m.wikipedia.org/wiki/Precision_Time_Protocol It does require hardware support, though.